This invention relates to an electrochemical electrode of the type used for ion activity measurements. Recently several ion sensitive electrodes containing certain macrocyclic compounds have been described in the literature and have become commercially available. A potassium ion responsive electrode employing, as its ion selective ionophore, a liquid organic sensing solution impregnated in a Millipore filter is described in an article by Pioda et al entitled "Highly Selective Potassium Ion Responsive Liquid-Membrane Electrode", Analytical Letters, vol. 2, pp. 665-674 (1969) and in U.S. Pat. No. 3,562,129. These publications describe the liquid organic sensing solutions as comprising either valinomycin, nonactin or monactin contained in diphenylether.
An article by Frant et al entitled "Potassium Ion Specific Electrode With High Selectivity for Potassium Over Sodium", Science, vol. 167, pp. 987-8 (1970) makes reference to a potassium ion measuring electrode having an ionophore comprising nonactin in Nujal-octanol, and to a commercially available electrode having a liquid organic ionophore comprising a mixture of valinomycin and an aromatic solvent. Examples of the solvents mentioned in the article are nitrobenzene and higher homologs, diphenylether, chlorobenzene, and bromobenzene.
Another commercially available potassium ion electrode employs a sensing solution containing a major portion of a non-aqueous hydrophobic solvent, such as decane and minor portions of valinomycin and the phospholipid, lecithin. Such sensing solution is supported between a pair of cellophane membranes in a sample measuring cell in which the sample contacts the outside surface of one of the membranes and KCl solution in which a silver chloride electrode is immersed contacts the outside of the other membrane.
As can be seen from the above summary of recent organic potassium ion measuring electrodes, liquid organic sensing solutions are employed which are either supported by a filter paper or a cellophane membrane. One purpose of our invention is to provide a potassium ion measuring electrode which employs an essentially solid ion sensitive barrier containing a macrocylic compound, which offers considerable ease of use over the liquid membrane electrodes, besides having excellent selectivity to the ion being measured.
More recently, U.S. Pat. No. 3,706,649 teaches a substantially solid ion sensitive barrier containing a macrocylic compound, mineral oil, and a lipid. This patent indicates that other solidification agents, i.e. collodian polystyrene, silica gel and colloidal silica when used in place of the lipid greatly diminished or eliminated the ion sensing properties of the solid ion sensitive barrier. In this arrangement it is necessary to put and hold in place a protective covering membrane having perforations therein over the ion sensitive barrier.
In U.K. patent application Ser. No. 2,025,629 A, published Jan. 23, 1980, there is disclosed an ion selective electrode assembly. A tube of plastic material defining a flow path through the electrode assembly contains in a portion of the tube wall an ion selective membrane bonded to the tube wall so that biological fluid slides past the membrane surface in a generally laminar flow.
In accordance with this invention, there is provided a disposable one-piece electrode system in which a half cell conductor is disposed in an electrolyte solution in a sealed chamber. A passage extends through one wall of the chamber to an external surface arranged for exposure at an angle to the flow of the sample solution to be analyzed. Disposed in that passage is a cast-in-place membrane with one surface essentially flush with the external surface of the chamber and an opposed surface that is in direct and intimate contact with the electrolyte solution. The half cell electrode may be placed closely adjacent the internal surface of the cast-in-place membrane such that the electrode system is compact, durable, reliable, maintainence free, virtually unbreakable and easily replaceable. The electrode system is placed in a fluid flow path so that the flow of the solution whose ion content is to be measured impinges directly on the cast-in-place membrane.
In a particular embodiment, the half-cell conductor is a silver-silver chloride wire; the electrolyte is sodium chloride-potassium chloride solution prepared as described herein and the cast-in-place membrane is about 1.5 millimeters in diameter; and an initial electrical resistance of about 10.sup.8 ohms and generally will not exceed 2.times.10.sup.9 ohms during the life of the electrode. The electrode unit has an overall length of about 3.5 centimeters and a diameter of about 1.5 centimeters.